1. Field of the Invention
The present invention relates to a resist pattern forming method and a resist material suitable for this method, and more particularly, it relates to betterment for improving dry etching resistance of a resist material.
2. Description of the Background Art
In recent years, a semiconductor device is increasingly refined followed by a shorter wavelength of light for irradiation employed for photolithography. A resist material employed for photolithography must be transparent with respect to light of a wavelength in an application range therefore. As base resins serving as the main components of resist materials satisfying such requirement, novolak resin (resin A described later) suitable to g to i rays (436 nm to 365 nm in wavelength) of a mercury lamp and polyhydroxystyrene (resin B described later) suitable to a KrF excimer laser beam (248 nm in wavelength) are widely known as typical resins at present.
On the other hand, acrylic resins C and D containing no benzene rings, illustrated in FIGS. 17 and 18, are now watched with interest as base resins for resist materials employed for an ArF excimer laser beam (193 nm in wavelength) which is regarded as advanced light for irradiation. A material such as novolak resin containing a benzene ring is unsuitable to an ArF excimer laser beam in view of transparency, despite an advantage of excellent dry etching resistance.
In each of the resins C and D shown in FIGS. 17 and 18, units U1, U2 and U3 form methacrylic units respectively. A resist material composed of a base resin prepared from the resin C or D having such methacrylic units contains an acid generator such as triphenylsulfonium triflate as a mixture which is added to the base resin.
When the resist material is irradiated with light, hydrogen (generally acid) is generated due to the action of the added acid generator, to act under heating on that substituent D1 in the resin C or D which is capable of being decomposed by the acid. Consequently, this substituent D1 is cut off from a principal chain of the polymer. Thus, the polymer is denatured by irradiation with light.
When a developer is applied to the resist material after the heating, parts of the polymer denatured by the light are readily dissolved while the remaining parts are not dissolved but remain. In other words, the parts of the resist material irradiated with the light are selectively removed. Consequently, patterning of the resist material is implemented. Namely, the resin C or D forms a positive chemical amplification resist material.
However, the resins C and D containing no benzene rings which are expected for application to an advanced resist material are inferior in dry etching resistance to a resin such as novolak resin which is widely employed at present. This bottlenecks reduction of the wavelength of the light for irradiation.
On the other hand, the resist aspect ratio (resist film thickness/resist pattern size) is increased as a result of improvement in resolution following reduction of the wavelength of the light for irradiation. In order to prevent the resist pattern from falling in the so-called resist pattern fall after development and rinsing, it is necessary to suppress the thickness of the resist material which is deposited on a target of etching (i.e., an underlayer) such as a surface of a semiconductor device.
Further, the etching rate depends on the resist aspect ratio, and known is the so-called RIE-lag such that the etching rate is retarded as the width of the resist pattern is narrowed, i.e., the aspect ratio is increased. In order to suppress such reduction of the etching rate caused by the RIE-lag phenomenon for preventing insufficient etching, it is as well necessary to suppress the thickness of the resist material.
As the thickness of the deposited resist material is suppressed, higher dry etching resistance is required to the resist material. In order to reduce the wavelength of the light for irradiation, therefore, the dry etching resistance, i.e., the selection ratio of the resist material to the underlayer in etching must be improved beyond that in the conventional resist material.